1. Field of the Invention
This invention relates generally to heating, ventilating and air conditioning (HVAC) systems and more specifically to an interface circuit for use in controlling the speed of a variable speed electrically commutated fan motor.
2. Brief Description of the Prior Art
Electrically commutated motors or ECM's are DC motors having internal inverters in which low voltage control signals are used to select the motor speeds. However, the use of such a motor in HVAC systems such as a heat pump blower assembly, for example, leads to certain problems. In HVAC systems there are many paths which can conduct current and since this type of motor used for the fan will run, albeit at reduced efficiency, on incomplete portions of a sine wave, prior art approaches have required various discrete components to avoid spurious signals from causing unintended energization of the motor. In a heat pump it is necessary to energize the fan motor whenever electric heat has been energized. This requires the detection of 240 VAC power being applied to the electric heating element and subsequently energizing the fan motor. Further, when a "continuous fan" mode of operation is selected the lowest fan speed, or an on/off signal to the ECM, must be the only signal energized.
A prior art method for accomplishing this employs a transformer for transforming the 240 VAC signal to 24 VAC which is used to drive the ECM control lines. However, it is necessary to isolate the low voltage signal from the continuous (low) speed fan operation from another low voltage signal calling for high speed fan operation. Improper isolation can result in high speed fan operation when the low speed fan is selected. This system, which uses an additional transformer for a heat detection circuit, is inherently costly using two separate transformers as well as being unreliable.
It is an object of the present invention to provide a circuit which does not have the above limitations of the prior art, a circuit which is of low cost and yet reliable for controlling the variable speeds of an ECM motor.